This invention relates to a turbine or progressive cavity apparatus, and more particularly to drive trains for downhole drilling devices and especially to such devices used in directional drilling.
One of the most common directional drilling techniques is the use of downhole drilling motors in combination with a bent sub. The bent sub is a section of drill pipe manufactured with a slight angle that is installed in the drill string above the bit. The built-in angle of the sub exerts a side force on the bit and causes it to be deflected from the previous direction of the hole. Bent subs typically provide deflections ranging from near zero to 5.degree.. Thus, the drill shaft must be capable of bending or articulating so that the downhole motor can pass the bend in the drill pipe. Typically, either turbine-type or progressive cavity downhole motors are used. However, it is also possible to use an electric motor in some cases.
The use of turbine-type downhole motors is well known in the art. In this type of equipment, the drill bit is rotated by an axial turbine power unit near the bottom of the hole. The turbo drill is driven by the circulating liquid, i.e., "mud", with which the drill hole is flushed. Since the driving motion is simple rotation, the use of turbine-type downhole motors presents no additional problems for directional drilling.
The use of progressive cavity or downhole motors is similarly well known. These devices have a single shaft in the shape of one or more helix contained within the cavity of a flexible lining of a housing. The generating axis of the helix constitutes the true center of the shaft. This true center of the shaft coincides with its lathe or machine center. Typically, the lined cavity is in the shape of a two or more helices (one more helix than the shaft) with twice the pitch length of the shaft helix. Either the shaft or the housing is secured to prevent rotation; the part remaining unsecured rolls with respect to the secured part. As used herein, rolling means the normal motion of the unsecured part of progressive cavity devices. In so rolling, the shaft and housing form a series of sealed cavities which are 180 degrees apart. As one cavity increases in volume, its counterpart cavity decreases in volume at exactly the same rate. The sum of the two volumes is therefore a constant.
When used as a motor for down hole drilling, the unsecured part or rotor produces a rotor driving motion. The driving motion of the rotor is quite complex in that it is simultaneously rotating and moving transversely with respect to the stator. One complete rotation of the rotor will result in a movement of the rotor from one side of the stator to the other side and back. The true center of the rotor will of course rotate with the rotor. However, in a typical construction, the rotation of the true center of the rotor traces a circle progressing in the opposite direction to the rotation of the rotor, but with the same speed (i.e., reverse orbit). Again, optimum performance is obtained when movement of the rotor is precisely controlled. One complete rotation of the rotor will result in one complete rotation of the true center of the rotor in the opposite direction. Thus, the rotor driving motion is simultaneously a rotation, an oscillation, and a reverse orbit For multi-lobe motors the reverse orbit is a multiple of the rotational speed, e.g., if a three lobe motor is used the reverse orbit is three times as great as the rotational speed.
Examples of progressive cavity motor and pump devices are well known in the art. The construction and operation of such devices may be readily seen in U.S. Pat. No. 3,627,453 to Clark (1971); U.S. Pat. No. 2,028,407 to Moineau (1936); U.S. Pat. No. 1,892,217 to Moineau (1932) and U.S. Pat. No. 4,080,115 to Sims et al. (1978).
The use of progressive cavity motors for directional drilling introduces additional problems. Specifically, despite the simple construction of progressive cavity devices, use of the devices as motors in apparatus has proven difficult. This difficulty results primarily from the failure to provide a drive train capable of handling the complex rotor driving motion (described above) in a durable, reliable and inexpensive manner. Couplings that connect the rotor of progressive cavity motors with the drill must be capable of operating in a contaminated, hostile environment while handling a very high torque and transmitting the rotational output of the rotor without the orbital motion of the rotor.
Attempts have been made to convert the complex rotor motion into rotational motion for driving a drilling shaft. Of the couplings which have been used in progressive cavity devices, the most commercially successful has been a universal joint attached to the driving end of the rotor and connected to a universal joint attached to the driven drill shaft. As is known, such U-joints react or resolve the orbital motion by the sliding of pin members in a universal assembly. Thus, such joints typically include elements which slide relative to one another.
The principle on which the Hooke's type of universal assembly works is illustrated in FIG. 3. The shaft A is formed into a fork or yoke at its end and pivoted between the prongs of this fork is a cross-piece C. The cross-piece C can therefore pivot about the XX relatively to the shaft A. The other shaft B similarly includes a fork or yoke at its end and the other arms of the cross are pivoted between the prongs of this fork. The shaft B can therefore pivot about the axis YY relative to the cross C and, since the latter can pivot about the axis XX relative to the shaft A, the shaft B can assume any angular position relative to shaft A. It follows that if the shafts A and B are supported in bearings with their axes at an angle, then when the shaft A is turned about its axis, the motion is communicated to the shaft B and it turns about its axis; the arms of the cross meanwhile oscillating in the prongs of the forks.
The axes XX and YY intersect at 0 and are perpendicular to one another. The axes of the arms of the cross C are also perpendicular to their respective shafts. The axes of the shafts A and B also intersect at 0, which point is commonly referred to as the "center" of the joint.
Although FIG. 3 shows a specific pivoting connection, it does not matter how the pivoting action is obtained. All that is required is that the shaft B shall be able to pivot independently about two intersecting perpendicular axes such as XX and YY, relatively to shaft A. There are many known constructions for achieving this result.
The single Hooke's type of universal assembly described above suffers from a disadvantage which is obviated in some other forms of the joint. Specifically, when two shafts are connected by a single Hooke's joint and one of these shafts is rotating at an absolutely constant speed, then the other shaft will not rotate at a constant speed but at a speed that is, during two parts of each revolution, slightly greater and, during the other two parts of the revolution, slightly less than the constant speed of the first shaft, i.e., the velocity varies cyclicly. The magnitude of this fluctuation in speed depends on the angle between the axes of the two shafts, being 0 when that angle is 0.degree. but becoming considerable when the angle is large. This disadvantage becomes of practical importance in applications such as downhole drilling when it is important to maintain a constant or substantially constant speed. The disadvantage can be obviated by using two Hooke's joints arranged with an intermediate shaft arranged so that it makes equal angles between the first and second stub shafts and the pivot axes of the intermediate shaft being arranged parallel to each other. The irregularity introduced by one joint is then cancelled out by the equal and opposite irregularity introduced by the second joint.
Attempts to apply universal joints to downhole motors have suffered from several disadvantages, particularly in the area of reliability. The primary reason for this is that the fluids used in progressive cavity drilling apparatus often are or quickly become abrasive. This abrasive fluid flows between the relative moving (sliding) surfaces of the U-joint causing rapid wear.
In the past, there have been attempts to isolate the sliding pivot surfaces of a universal from contaminants or heavy vibrations. Examples of such constructions are shown in U.S. Pat. No. 2,727,370 to Holland; U.S. Pat. No. 3,262,284 to Maxwell-Holroyd; U.S. Pat. No. 3,545,232 to Neese et al.; and U.S. Pat. No. 4,861,314 to Mazziotti. However, in such known cases there is always sliding between the seal and one of the surfaces of the U-joint components. As a result of this sliding, the seal is not truly hermetic and the U-joint components are not perfectly isolated. Thus, the possibility of contamination exists, particularly in a high pressure application such as down hole drilling.
Another type of universal joint assembly for use in downhole motor drives is disclosed in U.S. Pat. No. 4,772,246 to Wenzel. This patent discloses a pressure equalization arrangement which significantly reduces the pressure differential across the seal. As a result, the likelihood of leakage of drilling mud into the universal joint is reduced. Despite the advantages it offers, this construction is complicated and expensive. Further, the U-joint components are not perfectly isolated because the seal is not hermetic. Consequently, there is some possibility of contamination of the U-joint assembly. Thus, while the need to seal, to some extent, the components of a U-joint has been recognized, the need to perfectly isolate these components and a reliable means of achieving this are not known in the prior art.
These problems are addressed in the present inventor's previous patent application Ser. No. 07/540,682 entitled "Progressive Cavity Drive Train With Elastomeric Joint Assembly For Use in Downhole Drilling" filed Jun. 20, 1990 and now allowed; Ser. No. 07/540,692 entitled "Downhole Drilling Apparatus Progressive Cavity Drive Train with Sealed Coupling" filed Jun. 20, 1990 and now allowed; and Ser. No. 07/540,693 entitled "Hermetically Sealed Progressive Cavity Drive Train For Use In Downhole Drilling" filed Jun. 20, 1990 and now allowed.
While a well sealed double universal joint is suitable for use downhole it is not necessarily ideal. There are very specific requirements for the coupling used downhole. An ideal coupling meets this requirement without sacrificing durability. An important factor to consider is that the coupling does not have to be terribly flexible. In normal use the coupling undergoes 3.degree. to 4.degree. deflection and thus, need not deflect more than 5.degree.. Thus, the flexibility offered by universal joints really isn't needed. In some cases a relatively thin shaft, i.e., a flexible shaft, can bend the necessary 5.degree. without a coupling. However, there are other requirements.
Regardless of whether the downhole motor is a turbine-type, a progressive cavity type or another type (e.g., electric) the shaft must bend up to 5.degree. to allow directional drilling. In order to do so, the shaft must be capable of transmitting enormous thrust and torque forces. For example, in a medium sized mud motor, the shaft must be capable of reliably transmitting a thrust load of 20,000 lbs. and a torque of 60,000 inch pounds. Conventional drill shafts are not capable of accommodating such loads for sustained periods. Thus, the drill shaft must be modified to include an articulated coupling which allows bending without sacrificing load carrying ability. The design of such an articulated coupling has presented a number of problems. These problems result from a simple paradox. Specifically, if the coupling on the drill shaft is wide enough to withstand the torque load which must be transmitted, it is too big to bend the necessary 3.degree. to 4.degree.. Conversely, if the drill shaft is thin enough to bend 3.degree. to 4.degree., it is too thin to withstand the enormous torque load which must be transmitted. In addition, in a progressive cavity motor, the shaft must accommodate the orbiting motion of the rotor. For a turbine drive the coupling would only need to accommodate the bend since there is no offset rotor.